A static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This static magnetic field is referred to as the B0 field. It is commonly known that increasing the homogeneity of the B0 field used for performing an MRI scan increases the quality of the diagnostic images, which benefits physicians using an MRI image to diagnose a patient.
To increase the homogeneity of the B0 the field in the imaging zone or region where the magnetic resonance imaging data is acquired is decomposed into spherical harmonics. The majority of magnetic resonance imaging systems have cylindrical symmetry, but the spherical harmonics have a closed form. Individual coils are designed to adjust or shim a single spherical harmonic of the B0 field. The first order components of the spherical harmonics are typically shimmed using the magnetic field gradient coils and the higher order terms are shimmed using dedicated shimming coils. The magnetic field gradient coils create gradients in the magnetic field in order to spatially encode the radio frequency signals that the nuclei emit during magnetic resonance imaging.
The shimming coils in magnetic resonance imaging systems occupy the high magnetic field region which is adapted for receiving a subject. For example, magnetic resonance imaging magnets with a cylindrical geometry are placed within the bore of the magnet, and each shim coil occupies an individual layer. As a result the shim coils reduce the useable portion of the magnet.
U.S. Pat. No. 7,427,908 B1 discloses the combination of the corrective functionality of the standard X and ZX shim coils into two standard, simplified electrical circuits and conductors. U.S. Pat. No. 7,427,908 B1 also discloses a similar combination of the Y and ZY shimming coils. Coils in these arrangements overlap orthogonally, which allows a reduction in the radial space needed for shimming.